Steve,
The stock style synchronizers in a 930 box call for the use of a dino type gear oil as synthetics are too slippery and will prevent them doing their job properly. Dino oil has a proprensity to thermal viscosity break down. Putting 500 HP through this box, and especially the crown & pinion gears, is going to stress them and generate a lot of heat. So, running a cooler is kind of like buying an insurance policy. Yes, you can get by without it, but it is really nice to have that added margin of safety. The choice is yours.
Lynn